1. Field of the Invention
The invention relates to a neutron activated heat source, in particular, an isotopic heat source using the isotope thulium-170.
2. Description of Related Art
Isotopic heat sources use the release of energy from a radioactive isotope. The isotope is created either as a result of fission or by irradiating a target material with neutrons in a nuclear reactor. In neutron irradiation, the target atomic nuclei capture irradiating neutrons and are converted into a neutron activated isotope. The target material is chosen to provide the energy release rate and decay characteristics of interest in the activated target. This energy release can be absorbed as heat and exploited for many uses, such as for a power conversion system.
Typically, reactor target materials are formed into thin flat disks. During irradiation, neutrons are highly absorbed at the target surface, resulting in fewer neutrons available for absorption in the center of the target. The reduction in neutrons, called flux depression, results in lower activation in the target center compared to the target surface. Thin targets provide a more efficient use of target material by reducing flux depression.
Targets may contain a material that acts as a moderator during irradiation. Neutrons that pass through the target atoms unabsorbed can collide with moderator atoms, slow down, and become more susceptible to capture by other target nuclei. Moderators thereby increase the efficiency of the production of the activated isotope. An ideal amount of moderation causes the neutron energy distribution to peak in the energy region of high cross-section for the target material.
Isotopic heat sources are useful when combined with a power conversion system because the energy release is reliable, and the power output diminishes in a known manner as the isotope decays. The heat sources have greater energy density, by several orders of magnitude, than chemical batteries. Depending on the half-life of the isotope, the heat sources can be used for months or years, rather than having a life of hours or weeks that is typical of a chemical battery. The sources are compact and portable, which is especially useful for exploration or surveillance in remote areas such as Antarctica, in space, or underwater.
Presently, isotopic heat sources are available that use isotopes such as strontium-90, cobalt-60, and plutonium-238. These isotopes are environmentally hazardous because they are easily dispersed, and their half-lives are on the order of years.
Thulium-170 has also been considered as a fuel for heat sources. Targets with stable thulium-169 are irradiated and converted into thulium-170 (and thulium-171, etc.). Thulium-169 has a high neutron cross-section, lowering the irradiation time (and cost) needed to produce thulium-170. Thulium is advantageous as a fuel because of its refractory properties; that is, thulium is very stable at high temperatures and has a high melting point (heat of fusion). Thulium-170 is a better heat source from an environmental standpoint because of its relatively short half-life (129 days), its chemical stability, and refractory nature.
Several isotopic heat sources using thulium-170 have been developed. The thulium fuel has been in the form of thulium hydride, thulium metal, thulium oxide, and a mixture of thulium oxide and thulium metal. The thulium fuel is usually encapsulated or encased in a material with a high melting point and low neutron cross-section. These materials are usually metals or high atomic weight (high Z) materials, such as molybdenum, tantalum, tungsten, zirconium, steel, nickel, or platinum-rhodium alloy. The casings provide containment of the target material before and after irradiation.
Using high Z material to encapsulate targets presents several problems: the heat source weight is increased, pre-fabrication of the capsules is needed, and high Z materials produce more bremsstrahlung radiation after target irradiation than low Z materials. Accordingly, a more useful heat source would comprise a refractory fuel with a short half-life and a diluent of low atomic weight (low Z) material. The low Z material would reduce the weight of the heat source. The low Z material would also produce less bremsstrahlung radiation than a high Z material, requiring less shielding. The reduction in shielding and source weight is advantageous in creating portable power sources. Individual thulium fuel parts would not be encapsulated, minimizing pre-fabrication time and expense. Suitable containment would be provided by an outer vessel containing all of the thulium fuel parts.